The Sanden Eco® Plus heat pump hot water (HPHW) system operates like a refrigerator in reverse. It contains a fan that forces air through an evaporator that contains a refrigerant. The heat in the ambient air passes through the evaporator (note: even air below 0 º C contains heat energy) and is absorbed by a natural refrigerant, R744 (aka CO2), which is ozone friendly and does not significantly contribute to global warming compared with most other refrigerants (older style refrigerants have thousands of times more global warming impact for the same amount of gas, when compared with CO2).
The warm gaseous refrigerant is circulated in the system via a compressor. As it passes through the compressor, its pressure rises, as does its temperature. This hot refrigerant then passes through a heat exchanger to heat the water, which is finally pumped to the storage tank.